Rotary X-ray tube target



Nov. 5, 1963 R. F. KASTEN, JR 3,109,951

ROTARY X-RAY TUBE TARGET Filed Dec. 2, 1960 INVENTOR. Roy E Kasfen, Jr.

Buckhorn, Cheafham a Blore ATTORNEYS United States Patent 3,109,951 ROTARY X-RAY TUBE TARGET Roy F. K asten, Jr., Chicago, Ill., assignor to Dunlee Corporation, Bellwood, 11]., a corporation of Illinois Filed Dec. 2, 1960, Ser. No. 73,333 2 Claims. (Cl. 313-55) Pa'lhe 1prtleseitit invention relates to X-ray tubes and more 1 r-icuary o a newand' anode Xiay tube. improved design of a rotary Among others, two very critical problems have been encountered in attempting to increase the operating po tential and ratings 'of rotary anode X-ray tubes. One of these problems resides in the dissipation of the heat generated at the anode target area. Unless the heat generated by the impacting electrons is removed sufliciently rapidly, the temperature of such areas may exceed the melting point of the target material whereupon it will flow or crater effectively rendering the tube inoperative. Most generally the target material is tungsten which has been used by itself in relatively thin, dished plates and in which form the problem of overheating has occurred. In efiorts to solve the heat dissipation problem the tungsten target has been formed as a laminae upon a copper backing, it being considered that the copper would conduct the heat away from the tungsten suificiently rapidly to prevent overheating of the target. However,

the tungsten and the copper tend to delaminate whereupon the tungsten peels away fr om the co to destroy the target surface conformation. liheii f ofr ri d solely of tungsten the dished target members have had in tubes of standard geometry, i.e., with a target member of about 2% inch diameter, a thickness no more than about 4 inch. With such targets it has been necessary to hm1t the load thereon in order to avoid overheating With target members of lesser diameter the capacity is gglrl lbrirore restricted because :of the lesser mass of the The other critical problem which has been present in attempting to increase the operating potential of rognodle tugs; existed in the high potential gradient 1c eve o e etwee v and the tube epnvelopa n the ed e of the target member It is my discovery that the ca acit of anode X-ray tube may be increased uneiipectedly b y ti l slmple expedient of providing a tungsten target member of greater thickness than used heretofore and of providing such target member with an edge which in cross section is semi-circular.

It is, accordingly, an object of the present invention to provide a new and improved rotary anode X-ray tube.

More particularly, it is an object of the present invention to provide a rotary anode X-ray tube having increased operating capaci Another object is to provide a rotary anode target member for an X-ray tube that has improved characteristics for the dissipation of heat.

Still another object is to provide a rotary anode target member configuration that minimizes the potential gradient between the target member and the tube envelope.

A further object of the present invention is to provide a new and improved target member for a rotary anode X-ray tube which will enable an increase in the operating potential of the tube.

A more particular object of the invention is to provide a target member which will have greater heat capacity than those presently employed.

o h objects and advantages of the invention will become more apparent in the following detailed description of the invention and the accompanying drawings wherein:

FIG. 1 is an elevation partly in section of an X-ray tube made in accordance with the present invention;

FIG. 2 is an enlarged elevation of the target member looking in the direction of the arrows 22 in FIG. 1;

FIG. 3 is a sectional view through a target member taken along line 3-3 of FIG. 2; and

FIG. 4 is a similar sectional elevation of a target member constructed in accordance with the prior art.

Illustrated in FIG; 1 is a rotary anode type X-ray tube 10, comprising an evacuated envelope 12 of glass or other suitable material. Sealed in one end of the envelope by suitable means is a cathode assembly 14 including a filament 16 adapted upon connection to a suitable source of electrical energy to generate a stream of electrons 18 which are focused upon a plate like target member 20 of an anode assembly 22 mounted in the opposite end of the envelope. The anode assembly includes a suitably mounted rotor 24 adapted to be driven by a stator (not shown). Projecting from the rotor 24 is a stem 26 upon which the target member 20 is mounted. A nut 28 or other suitable means may be provided to hold the target member in place upon the stem 26. Upon application of potential between the cathode 14 and anode 22 the electrons generated at the filament 16 are accelerated toward the anode and strike the target member in a focal spot 30. Since the target member rotates when the tube is in operation, an annular focal area of the target member indicated 'by the dotted lines 32 will rotate progressively into the path of the electrons 18.

As shown in FIG. 3, the target member 20 is concavo-convex in configuration, that is, it is dish shaped with the frusto conical focal spot area inclined at the conventional line focus angle of about 15 degrees as shown in the drawing, and is of uniform thickness t throughout except at its peripheral edge. The thickness of the target member may if desired be reduced in the immediate area of the nut 28 to reduce material cost. In accordance with the invention the peripheral edge preferably is semi-circular in configuration, the radius of curvature r being equal to one half the thickness t. As has been indicated previously, heretofore solid tungsten disc targets have been of 71 inch thickness or less. With such thicknesses the operating capacity of the X-ray tubes has been limited because of the tendency of the target disc to overheat if too great a load was imposed thereon. In addition, because of the target edge configuration to be described subsequently, the tubes have had a poor electrical gradient between the target and the tube envelope and have had a tendency to be electrically unstable. It is my discovery that an unexpected increase in the operating capacity of a rotary anode X-ray tube can be obtained by the simple expedient of rounding the target edge so that in cross section it is semi-circular and increasing the thickness of the tungsten disc. For a target diameter of about 2% inches I have found the minimum target thickness should be 4 inch. The maximum practical target thickness for such diameter is about /2 inch. For smaller target diameters somewhat greater thicknesses are desired so as to provide the necessary mass and heat capacity. As an example of the surprising improvement in tube performance I found that by replacing a solid tungsten target 2% inches in diameter and inch thick with one of the same diameter but of /1 inch thickness, the permissible load (expressed as the product of potential in kilovolts and current in milliamps) for a thirty second exposure may be increased from 2500 to 5000 in tubes having the same geometry otherwise.

The formation of the target member edge with a full a) radius, that is, rounding the edge so that in cross section it is semi-circular as shown in FIG. 3 and of a diameter equal to the thickness of the target member, minimizes the potential gradient between the edge of member and the tube envelope. Heretofore little consideration has been given this factor in the design of the target member. A typical prior art target member 40 is shown in section in FIG. 4. As indicated, such target members were .formed with :a relatively flat edge surface 48 and presented a rather sharply rounded corner 50 toward the adjacent envelope wall, the radius of such corner usually being of the order of inch. As a practical matter such construction created problems because of the arcover between the target member and envelope that sometimes occurred as a result of the high gradients produced. With the greater radius provided by the target members made in accordance with the invention such difliculties do not occur because of the substantially lesser potential gradient between the tube wall and the target edge. Theoretic-a1 computation following the formula shown by Peck, Dielectric Phenomena in High Voltage Engineering, at page 25 and assuming the envelope to be at ground potential indicates that in a tube having 150 kvp. applied between the :anode and cathode and with a target member of inch thickness and a full semi-circular edge, the potential gradient between the target member and envelope will be about 200 kv. per inch. With a target member such as shown in FIG. 4 where the corner radius is inch the potential gradient would be about 600 kv. per inch. Gradients on the order of the latter tend to render a tube unstable.

Having illustrated and described a preferred embodiment of the invention, it should be apparent to those skilled in the art that the invention permits of modification in arrangement and detail. I claim as my invention all such modification as come within the true spirit and scope of the appended claims.

I claim:

1. An X-ray tube comprising a cathode and a rotary anode and an evacuated envelope enclosing the same, said anode comprising a circular platelike, solid tungsten target member having a thickness of inch and a diameter of about 2% inch. I

2. An X-ray tube comprising a cathode and a rotary anode and an evacuated envelope enclosing the same, said anode comprising a circular, dish shaped solid tungsten target member, and means supporting said member for rotation about its axis, said target member being of substantially uniform thickness except at the peripheral portion thereof, said peripheral portion being semicircular in cross-section, the diameter of curvature of said peripheral portion being equal to said thickness, said thickness being inch and the diameter of said target being about 2% inch.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Files et al.: Practical Considerations in the Use of the Rotating-Anode Tube, reprint from the X-Ray Technician, September 1941, vol. 13, page 60 (8 pages; page 3 relied on). 

1. AN X-RAY TUBE COMPRISING A CATHODE AND A ROTARY ANODE AND AN EVACUATED ENVELOPE ENCLOSING THE SAME, SAID ANODE COMPRISING A CIRCULAR PLATELIKE, SOLID TUNGSTEN TARGET MEMBER HAVING A THICKNESS OF 1/4 INCH AND A DIAMETER OF ABOUT 2 7/8 INCH. 